Neisseria meningitidis is a Gram-negative bacterium and the causative agent of meningococcal meningitis and septicemia. Its only known host is the human, and it may be carried asymptomatically by approximately 10% of the population (Caugant et al, 1994, Journal of Clinical Microbiology 32 323).
N. meningitidis may express a polysaccharide capsule, and this allows classification of the bacteria according to the nature of the capsule expressed. There are at least twelve serogroups of N. meningitidis: A, B, C, 29-E, H, I, K, L, W135, X, Y and Z, of which serogroups A, B, and C cause 90% of meningococcal disease (Poolman et al, 1995, Infectious Agents and Disease 4 13). Vaccines directed against serogroups A and C are available, but the serogroup B capsular polysaccharide is poorly immunogenic and does not induce protection in humans.
Other membrane and extracellular components are therefore being examined for their suitability for inclusion in vaccines. Examples include the outer membrane proteins of classes 1, 2 and 3 (porin; encoded by por genes), and classes 4 (Rmp) and 5 (Opacity proteins; encoded by opa and opc genes).
However, to date, none of these candidates is able to induce complete protection, particularly in children (Romero et al., 1994, Clinical Microbiology Review, 7 559; Poolman et al., 1995, supra).
To create an effective vaccine, it is necessary to identify components of N. meningitidis which are present in a majority of strains, and which are capable of inducing a protective immune response (for example, bactericidal antibodies).
In this regard, reference is made to International Publications WO 99/24578, WO99/36544, WO99/58683 and WO99/57280, each of which is incorporated herein by reference and describe a number of candidate proteins that may be useful in vaccines to immunize against Neisseria meningitidis. 
In this regard, particular reference is made to International Publication WO99/31132 and Peak et al. 2000, FEMS Immunol. Med. Microbiol. 28 329, each of which is incorporated herein by reference and describe a novel surface antigen isolated from a number of different strains of N. meningitidis, which surface antigen, and allelic variants thereof, for the purposes of this specification will be referred to as NhhA.